In general, a computerized scale of the foregoing type includes a series (e.g., ten) of so-called weighing buckets each associated with an underlying load cell or other means for producing an electrical signal representative of the weight of product in the bucket. Located above each weighing bucket is a holding bucket which contains a quantity of the product to be packaged. During each cycle, each empty weighing bucket is filled with product by momentarily opening the overlying holding bucket and allowing the product to fall into the weighing bucket. The weight of the product dropped into each weighing bucket is substantially less than the total weight of product which subsequently is placed in each package by the packaging machine.
After all of the weighing buckets have been filled, microprocessor-based control circuitry responds to the weight signals produced by the different load cells, adds the weights in various combinations of weighing buckets and then selects the particular combination of buckets that meets the minimum statistical weight for the package to be filled while providing the least excess weight. The weighing buckets of that particular combination then are emptied and the product therein is delivered to the packaging machine to be deposited in the package. Only those weighing buckets previously emptied are refilled by dumping from their respective holding buckets during the succeeding cycle.
The potential accuracy of computerized scales has not been realized by prior art scales since it has been necessary to reduce the number of available buckets in a weighing cycle in order to update the tare weights of the buckets. Because of product buildup and other well-known problems, the tare weights of the weighing buckets may change over a number of weighing cycles. In order to compensate for dynamic changes in the tare weights of the weighing buckets, new tare weights must be periodically calculated.
Until now, to accomplish a tare weight calculation, the weighing buckets emptied during the last weighing cycle were identified and one of the buckets was not refilled in the following cycle. By not refilling this emptied weighing bucket, its weight in the next cycle would accurately represent its tare weight. Accordingly, the weight measured in the next cycle for the empty bucket was stored by the microprocessor-based control circuitry as the bucket's updated tare weight. In order to help insure the tare weights of all the buckets were periodically updated, prior systems typically kept track of the elapsed time since the last updating of the tare weight for each bucket. The system chooses the bucket whose tare weight should be updated in the next weighing cycle by identifying the bucket with the longest elapsed time.
Although the foregoing method adequately updated the tare weights of the weighing buckets, it unfortunately reduced the possible weight combinations in the tare weight cycle since a lesser number of combinations of buckets were available whose net weights could be combined to give a total weight within the desired range. In general, the unavailability of one bucket during each weighing cycle reduced the number of possible combinations by one half. Consequently, a significant degree of accuracy was sacrificed. Moreover, the overall speed of packaging was reduced since the number of reduced combinations increased the chance that no combination of bucket weights would give a weight total within the acceptable range; if no acceptable combination was found, a package was not loaded for the cycle, and the overall packaging speed was reduced.
In a particular example, for a system having ten weight buckets, the tare weight of one bucket is measured each cycle. Therefore, only nine buckets are available each cycle for providing weight combinations. If the tenth bucket was available each cycle, the possible number of weight combinations doubles to 1,023. But with nine buckets available, only 511 combinations are possible. It can be easily appreciated that such a reduction represents a substantial loss of accuracy.